This invention relates, in general, to air-borne vehicle tracking and guidance systems and is directed, in particular, to improving the directional accuracy of such systems by improving the electrical performance of radomes used in such systems.
Airborne vehicles, such as guided missiles, aircraft and the like, have their electronic equipment covered by a radome of dielectric material which provides mechanical protection for the equipment and contributes to the streamlining of the vehicle. Typically, the antenna system for transmitting and receiving RF energy is mounted (gimbaled) in the radome itself and is actuated to provide mechanical scanning by rotating (oscillating) over measured azimuth and elevation angles relative to the vehicle axis, i.e., its direction of travel. This angle is called look angle. The direction of travel of the vehicle in response to the target information received by the antenna through the radome relative to true target location is called boresight error.
Boresight error is introduced into the tracking system due to the radome shape, material, frequency of operation and the polarization of the RF energy being received or transmitted by the antenna within the radome. The radome electrical performance is sensitive to the polarization of RF energy impinging thereon and is a major detrimental characteristic of the radome. Radome boresight error can be represented by a vector which in turn is resolved into two components for analysis and test purposes as shown in FIG. 1 which shows an outline of a typical existing streamlined radome 10.
The two components of boresight error are identified as inplane 12 and crossplane errors 14. The inplane error 12 is in the plane of rotation of the antenna center line 16 within the radome and crossplane error 14 is perpendicular to the inplane error as shown in this Figure. The magnitude of these errors vary as a function of radome roll, look angle, frequency, and polarization. Components of radome error are generally today reduced by the use of lenses, wall thickness variations, inductive plates, etc. and the performance characteristics of a typical stream-lined missile radome, such as shown as in FIG. 1, are shown in the graphs, FIGS. 2a and 2b.
In these figures, inplane error 12 and crossplane error 14 are plotted in the Y axis direction with the look angle Plotted in the X axis. The solid curved lines and the dashed lines show a 120.degree. swing of the antenna or 120.degree. look angle. The solid curve is for -45.degree. cross polarization and the +45.degree. is the dashed line.
It can be seen from these figures that crossplane data changes significantly with RF energy polarization while the inplane data is nearly polarization insensitive. The crossplane error polarization delta at a given radome look angel can result in a 2.degree. error that changes at the rate of the varying polarization. This large an error can be detrimental to a tracking system accuracy and reliability.
From the foregoing it can be seen that a primary object of this invention is to reduce boresight error as a result of the polarization of RF energy being received or transmitted by the antenna within the radome and thus improve the directional accuracy of tracking and guidance systems.